1. Field of the Invention
The present invention relates to a reversible thermosensitive coloring recording method of producing multi-color images by use of a reversible thermosensitive coloring recording medium which is capable of developing and decolorizing a multi-color image repeatedly by utilizing a coloring reaction between an electron-donor coloring compound and an electron-acceptor compound; a method of initializing the reversible thermosensitive coloring recording medium; the reversible thermosensitive coloring recording medium; and a recording apparatus for producing multi-color images by use of, or initializing the reversible thermosensitive coloring recording medium.
2. Discussion of the Background
Conventionally, thermosensitive recording media utilizing a coloring reaction between an electron-donor coloring compound (hereinafter, referred to as a coloring agent) and an electron-acceptor compound (hereinafter, referred to as a color developer) are widely known and have been employed in various fields, for instance, for use with terminal printers for computers, facsimile apparatus, automatic ticket vending apparatus, printers for scientific measuring instruments, and printers for CRT medical measuring instruments. However, such conventional thermosensitive recording media for use with the above-mentioned products do not have reversibility with respect to the coloring or decolorizing in image formation, so that the color development and decolorization cannot be alternately performed repeatedly.
Several thermosensitive recording media which can reversibly develop and decolorize or erase colored images utilizing a coloring reaction between a coloring agent and a color developer have been proposed. For example, a thermosensitive recording medium using the combination of phloroglucinol and gallic acid as color developers is disclosed in Japanese Laid-Open Patent Application 60-193691. Colored images developed by using the gallic acid and phloroglucinol upon the application of heat thereto are erased when coming into contact with water or aqueous vapor. In the case where this type of thermosensitive recording medium is employed, there are difficulties in imparting water-resisting properties to the recording medium and obtaining stable recording preservability. Furthermore, there is another problem in that a large image erasing apparatus is required to erase the displayed image on the above-mentioned recording medium.
Japanese Laid-Open Patent Application 61-237684 discloses a rewritable optical information recording medium which employs a compound such as phenolphthalein, thymolphthalein or bisphenol as a color developer. In the above optical information recording medium, colored images are formed by applying heat thereto and gradually decreasing the temperature thereof. The colored images can be decolorized or erased by applying heat to the recording medium at a temperature higher than the image developing temperature, and then by rapidly cooling the recording medium. In the case of this optical information recording medium, the color developing and decolorizing steps are complicated and the contrast of the colored images is not satisfactory with some undecolorized colors remaining on erased images.
Japanese Laid-Open Patent Applications 62-140881, 62-138568, and 62-138556 disclose thermosensitive recording media using a homogeneously dissolved composition of a coloring agent, a color developer and a carboxylic acid ester. The above recording media can assume a completely colored state at a low temperature, a completely decolorized state at a high temperature, and can maintain the colored state or the decolorized state at a temperature midway between the above-mentioned low temperature and high temperature. When heat is applied to the recording media using a thermal head, a white image (decolorized image), which is similar to a photographic negative, is recorded on a colored background, so that the usage of the above recording media is limited. It is also necessary that the temperature of the recording media be maintained within a specific range in order to preserve recorded images on the recording media.
Japanese Laid-Open Patent Applications 2-188294 and 2-188293 respectively disclose a thermosensitive recording medium utilizing a salt of gallic acid and a higher aliphatic amine, and a thermosensitive recording medium utilizing a salt of a bis(hydroxyphenyl)acetic acid or butyric acid and a higher aliphatic amine. These salts have a reversible color developing function and decolorizing function. Specifically when these salts are thermally decomposed to liberate the above-mentioned amines, those amines have a decolorizing function. With this type of recording media, a colored image can be developed in a specific temperature range with the application of heat thereto, and can be decolorized or erased by applying heat thereto at a higher temperature than the above-mentioned specific temperature range. However, since the color developing function and the decolorizing function are competitively effected, it is difficult to thermally control these functions by changing the temperature of the recording medium. Therefore, it is difficult to obtain a stable image contrast.
As mentioned above, the conventional reversible thermosensitive recording media utilizing the coloring reaction between a coloring agent and a color developer have many problems and are unsatisfactory for use in practice.
Conventionally, there has been a large demand for a multi-color recording medium. Recently, a reversible thermo-sensitive coloring recording medium capable of forming images thereon in two colors has become usable in practice, and in fact used for labels, coupon tickets, label sheets with an adhesive layer and a release backing paper applied thereto, and video printers. The above-mentioned reversible thermosensitive recording medium is fabricated by laminating a high temperature coloring layer and a low temperature coloring layer on a support. These coloring layers respectively produce a different color by the application of a different amount of energy.
For producing two different colors by use of the above reversible thermosensitive recording medium, two methods have been proposed.
In one method, a colored image produced in the low temperature coloring layer is not decolorized when a colored image is produced in the high temperature coloring layer, so that the resultant image produced in the reversible thermosensitive recording medium has a mixed color of the color produced in the low temperature coloring layer and the color in the high temperature coloring layer.
In the other method, when a colored image is produced in the high temperature coloring layer, a colored image produced simultaneously in the low temperature coloring is erased by use of an appropriate decolorizing agent.
In the former method, however, if the color produced in the high temperature coloring layer cannot sufficiently conceal the color produced in the low temperature coloring layer, two-colored images with a sufficiently high contrast for practical use cannot be obtained. For instance, when the color produced in the low temperature coloring layer is black, it is impossible to obtain two-colored images.
On the other hand, in the latter method, any combination of colors can be employed. However, an appropriate decolorizing agent for the above-mentioned purpose, capable of satisfying the requirement for the color development and decolorization of the recording medium, has not been found.
In addition to the above, it is difficult to obtain a multi-color or full-color reversible thermosensitive coloring recording medium capable of producing three or more colors, and a satisfactory reversible thermosensitive coloring image formation method has not yet been developed.
As mentioned above, there is a large demand for a multi-color recording medium, and it is considered that a practically usable multi-color recording medium offers substantial market potential.
However, studies on a reversible thermosensitive recording medium, particularly on a multi-color reversible thermosensitive recording medium, have just been started. It is generally considered that it will be difficult to obtain a multi-color image on a reversible thermosensitive recording medium by the conventionally employed method.
For instance, attention is paid to a thermal display using a metal-complex-based thermochromic material as being a simple display medium on which images can be reversibly written and erased. However, the thermal display has a problem in image contrast and is not satisfactory for use in practice. In addition, a thermal display medium utilizing the changes in the transparency of an organic compound depending upon the temperature thereof has been proposed. The thermal display medium, however, forms only black and white images, so that it is not suitable to use it as a display which is required to have a visual appeal or as an electron black board.